1. Field of the Invention
The present invention relates to an arc welding process using a consumable electrode. In particular, the present invention relates to a welding current controlling method for reducing the spattering of molten metal by sharply decreasing the welding current immediately before a new arc is established.
2. Description of the Related Art
FIG. 5 illustrates a conventional arc welding process using a consumable electrode or welding wire. The figure includes a diagram showing waveforms of welding current Iw and welding voltage Vw, while also including a diagram showing the metal transfer of the welding wire in a plastic state. As seen from the figure, the welding process comprises an alternate repetition of a short-circuiting time or period Ts and an arc generating time or period Ta.
More specifically, Waveform (A) in FIG. 5 shows the change of the welding current Iw with time (the current Iw passes through the welding wire 1), while Waveform (B) shows the change of the welding voltage Vw with time (the voltage Vw is applied between the contact-tip for power supply and the base material 2). Graphics (C) through (E) illustrate the shape-shifting of the molten wire end or droplet 1a. 
The short-circuiting period Ts, which ranges from time instant t1 to t3, is a stage where the molten droplet 1a at the tip of the welding wire 1 comes into contact with the base material 2, thereby establishing a short-circuiting state.
During the period Ts, the welding current Iw gradually increases, as shown in Waveform (A), while the short-circuiting state keeps the welding voltage Vw at a low level, such as a few of volts, as shown in Waveform (B).
The short-circuiting period Ts starts at time instant t1 when the molten droplet 1a contacts with the base material 2, as illustrated in Graphic (C). Subsequently, as illustrated in Graphic (D), the welding current Iw passes through the molten droplet 1a, and this produces an electromagnetic pinch force to cause the molten droplet 1a to deform or “neck down” at an upper portion or constriction 1b. Then as illustrated in Graphic (E), the constriction 1b is narrowed rapidly to cause the molten droplet 1a to be disconnected from the end of the welding wire 1 and to drop into the molten pool 2. Upon this, a new arc 3 is generated (this is referred to as “regeneration of arc” or “arc regeneration” below).
Next, the transition from the short-circuiting period Ts to the arc generating period Ta is described.
The period of time from the generation of the constriction to the arc regeneration is very short, such as, a few hundreds of microseconds. Therefore, the detection of a constriction makes it possible to tell the short-circuiting stage is to finish soon.
The detection of a constriction can be performed by monitoring the change in resistance between the welding wire 1 and the base material 2 during the short-circuiting period Ts. A generated constriction narrows the flow path for the welding current Iw, increasing the resistance of the constricted part. The more the part constricted, the more the resistance rises. Thus, to detect a constriction, the change of the resistance is calculated by dividing the welding voltage Vw by the welding current Iw to be monitored.
The way of detection can be more simplified with some approximation. As the first way of simplified detection, change of welding voltage Vw is just monitored instead of the resistance because the above-mentioned time frame, during which the constriction exists, is extremely short enough to ignore the change of the welding current Iw during the time frame as shown in Waveform (A). The detection of a constriction is performed by, first, calculating the rate-of-change (i.e. differential) of the resistance or the welding voltage Vw in the short-circuiting period Ts, and then, checking if the calculated rate-of-change reaches a predetermined value or threshold for necking detection.
As the second way of simplified detection, the detection of a neck is performed by acquiring a voltage increment ΔV in Waveform (B) from the stable short-circuiting voltage Vs, which can be obtained in the short-circuiting period Ts except the period after the constriction generates. Then, it is checked if the voltage increment ΔV reaches a predetermined neck detecting value or threshold Vtn at time instant t2.
The description below is made for the second detection method of a constriction. However, the first method or other detection methods may be employed for detecting a constriction.
At the end of the transition, it is required to detect regeneration of an arc at time instant t3. The detection of arc regeneration is simply performed by monitoring if the welding voltage Vw exceeds the judgmental boundary voltage Vta, which separates an arc generating stage from a short-circuiting stage. Specifically, if Vw<Vta holds, this indicates the short-circuiting period Ts. On the other hand, if Vw≧Vta holds, this indicates the arc generating period Ta. Hereinafter, the above-mentioned period of time from the time instant t2 (at which the generation of the constriction occurs) to the time instant t3 (at which the arc regeneration occurs) is referred to as “neck detecting period Tn”.
Referring to the last half period of FIG. 5, the arc generation period Ta, which starts at the time instant t3, is a stage where arc heat melts the tip of the welding wire 1 to produce a molten droplet 1a and also melts the base material 2. At the time instant t3, the welding current Iw starts to gradually decrease as shown in Waveform (A), whereas the welding voltage Vw surges to an arc generating voltage of around a few tens of volts as shown in Waveform (B).
Generally, an arc welding process using a consumable electrode employs a constant-voltage power source for welding. Further, the arc welding can be divided into two modes: a short-circuiting transfer, which occurs when the average value of welding current (or wire feeding speed) is low, and a globular transfer, which occurs when the average value of welding current is high.
In performing welding process with the short-circuiting state, the current value Ia at the time instant t3, at which an arc is re-generated, tends to become large. This causes large growth of the pressure (i.e. arc force) produced by the arc 3 to the molten pool 2a, resulting in generation of a large amount of spatter. The amount of generated spatter is substantially proportional to the welding current value Ia at the moment of arc regeneration. Thus, for prevention of spatter generation, the welding current need be set to a small value Ia at the moment of arc regeneration. Conventionally, various methods have been proposed to restrict the welding current value Ia at the moment of arc re-generating, including some methods by detecting a constriction and subsequent decreasing the welding current Iw sharply. Such conventional welding current controlling methods will be described below.
FIG. 6 shows an example of above-mentioned conventional welding current controlling methods employed in an arc welding process using a consumable electrode in order to restrict generation of spatter. The figure is a block diagram of a power source which the method employs to utilize its current control function upon detecting a constriction. It should be noted that blocks for wire feeding are omitted from the figure.
The power source main circuit MC receives commercial power supply such as three-phase alternating current with 200V as input, then executes some processing, such as inverter control or phase control using thyristors, based on the amplified error signal Ea which will be described later, and finally outputs the output voltage and the welding current Iw.
The current path for the welding current Iw includes a transistor TR and a resistor R which are inserted parallel to each other in order to control the current. At the moment that a constriction or neck is detected, as described later, the transistor TR turns off while the resistor R only permits the current to pass through and thus the welding current Iw is reduced sharply. The welding wire 1 is fed at a constant speed to generate an arc 3 with the base material 2.
The neck detecting circuit ND receives the welding voltage Vw as an input, detects a constriction by the method described, and then outputs the neck detection signal Nd, which keeps LOW-level during the neck detecting period Tn.
The driver circuit DR outputs the driving signal Dr, which turns the transistor TR Off only while the neck detection signal Nd is LOW-level. This operation enables the resistor R to function whereby the current path for the welding current Iw has an increased resistance more than ten times as large, resulting in sharp reduction of the welding current Iw. In periods except the neck detecting period Tn, the transistor TR is kept on and then makes the resistor R short-circuited, causing the power source in the default configuration.
The delay period setting circuit TDR outputs the delay period setting signal Tdr which is predetermined.
The uprise period setting circuit TUR outputs the uprise period setting signal Tur which is predetermined.
The first current level setting circuit IMR outputs the first current level setting signal Imr which is predetermined.
The second current level setting circuit IHR outputs the second current level setting signal Ihr which is predetermined.
The current controller NIC, which controls the current upon detection of a constriction, receives the above-mentioned signals Tdr, Tur, Imr, Ihr and Nd, each as an input, and outputs the switching signal Sw for power characteristics and the current setting signal Ir, both of which are described later with reference to FIG. 7.
The voltage setting circuit VR outputs the voltage setting signal Vr which is predetermined.
The current detecting circuit ID monitors the welding current Iw and outputs the current detection signal Id.
The voltage detecting circuit VD monitors the output voltage and outputs the voltage detection signal Vd.
The voltage-error amplifier EV amplifies the error between the voltage setting signal Vr and the voltage detection signal Vd, and then outputs the amplified voltage-error signal Ev.
The current-error amplifier EI amplifies the error between the current setting signal Ir and the current detection signal Id, and then outputs the amplified current-error signal Ei.
By using the switching circuit SW, it is possible to change the characteristics of the power source between two modes or two periods. The first one is a current-constant period, in which the circuit SW selects “a”-terminal path, that is a stage where the power source has current-constant characteristics. The second one is a voltage-constant period, in which the circuit SW selects “b”-terminal path, that is a stage where the power source has voltage-constant characteristics.
More specifically, the above-mentioned switching for power characteristics is performed by the switching circuit SW based on the switching signal Sw as an input. As described later with reference to FIG. 7, during the neck detecting period Tn, the delay period Td, and the uprise period Tu, the switching signal Sw causes the switching circuit SW to select “b”-terminal path, and hence select the amplified current-error signal Ei. On the contrary, during periods except the above-mentioned, the switching signal Sw causes the switching circuit SW to select “a”-terminal path, and hence outputs the amplified voltage-error signal Ev. Thus, the switching circuit SW outputs the selected one of the two signals as the amplified error signal Ea.
FIG. 7 is a timing chart concerning several signals in the power source shown in FIG. 6. Waveforms (A), (B), (C), (D), and (E) illustrate time change of the welding current Iw, the welding voltage Vw, the neck detection signal Nd, the switching signal Sw for power characteristics, and the current setting signal Ir, respectively. More details will be described below with reference of the figure.
In the figure, the current-constant period ranges from time instant t2 to t5. The rest is the voltage-constant period, where the waveforms of current and voltage are the same as the default ones shown in FIG. 5 with the transistor TR in an ON-state as described above.
At the time instant t2, the voltage increment ΔV reaches the neck detecting threshold Vtn as shown in Waveform (B), causing the neck detection signal Nd to turn LOW-level as shown in Waveform (C). This results in LOW-level of the switching signal Sw for power characteristics. At the same time, LOW-level of Nd turns off the transistor TR, causing the welding current Iw to reduce sharply as shown in Waveform (A) to the first current level Im, which is relatively low, indicating the generation of a constriction.
At the time instant t3, an arc is re-generated, causing the welding voltage Vw to reach the judgmental boundary voltage Vta, which separates an arc generating stage from a short-circuiting stage, as shown in Waveform (B). This results in return of the neck detection signal Nd to HIGH-level as shown in Waveform (C).
Generally, large welding current can generate spatter. Therefore, when an arc is re-generated, restricting the welding current can eliminate generation of spatter by the weakened arc force at droplet transfer. In addition, generally, spatter can be generated by resonance of the molten pool with change of arc force produced by change of the current. Therefore, when the molten pool is vibrating by droplet transfer, it is also effective for spatter elimination to wait for the vibration to be attenuated before increasing the welding current again.
For these reasons, in order to restrict generation of spatter, the delay period Td is provided preliminarily from time instant t3 to t4. The period is typically set less than 1 ms, depending on the welding condition.
During the delay period, the welding current Iw is kept to the first current level Im, which is relatively low, as shown in Waveform (A). This is performed by that the first current level setting signal Imr defines a value, and the value is set to the current setting signal Ir then Ir=Im is satisfied during the delay period Td as shown in Waveform (E).
At time instant t4, at which the delay period Td ends, the current setting signal Ir surges to a value which the second current level setting signal Ihr defines, as shown in Waveform (E). Ir keeps the value Ihr during the uprise period Tu, which ranges from time instant t4 to t5. During the period, the switching signal Sw keeps LOW-level as shown in Waveform (D), enabling the welding current Iw to surge to the second current level Ih, which is relatively high and indicates the arc generation, as shown in Waveform (A).
At time instant t5, the switching signal Sw turns HIGH-level as shown in Waveform (D), switching the power source characteristics to constant-voltage. Operations after time instant t5 are the same as operations in FIG. 5, and therefore further explanations are not given.
The above-described conventional methods are disclosed in JP-A-S59-206159 gazette and JP-A-H4-4074 gazette.
The conventional welding current controlling method, described above with reference to FIG. 7, has some disadvantages.
Firstly, it includes sharp surge of the welding current Iw at the time instant t4, which is the end of the delay period Td following an arc regeneration. This current surge causes rapid change of arc force, resulting in intense vibration in the molten pool to generate spatter. Secondly, the vibration in the molten pool may cause the tip of the wire to make contact with the molten pool, resulting in accidental occurrence of short-circuiting again. Different from normal and stable short-circuiting, this kind of accidental short-circuiting is unstable and cause to generate spatter easily.
In order to restrict generation of spatter, the above-mentioned current controlling method upon detecting of a constriction employs special circuit configuration which costs high. Therefore high performance which is worth with the high cost is required to prevent a constriction from generating.